The invention relates to a method for the separation of molten aluminum and solid inclusions formed from a melt of aluminum containing one or more foreign chemical elements.
During the production of aluminum one of the problems is that, depending on its subsequent use, too high a concentration of foreign chemical elements is present, both when the aluminum is produced from aluminum ore and when recycled aluminum is used. For instance aluminum scrap can contain too much Fe, Mn, Si or Mg for use for commercial purposes without mixing it with primary aluminum containing little of the foreign element. These foreign elements can be removed by cooling molten aluminum, resulting in the formation of inter-metallic compounds or crystals containing only one foreign element as solid inclusions when the molten aluminum containing the foreign elements is hypereutectic. If the molten aluminum containing the foreign elements is hypoeutectic, first foreign elements are added to make the melt hypereutectic.
When more than one foreign chemical element is present and the molten aluminum is cooled, first one inter-metallic compound or crystals containing only one foreign element is/are formed, and after that a second or even third or further inter-metallic compound or crystals will be formed.
These inter-metallic compounds or crystals are present in the molten aluminum and have to be removed to purify the molten aluminum. For the removal of the inter-metallic compounds or crystals several techniques are known, such as gravity sedimentation, flotation, filtration, centrifugation, electromagnetic sedimentation, and ultrasonic treatment. These methods offer an effective way to remove both small and large particles.
An important drawback of the known removal techniques is that quite a lot of molten aluminum is lost as interstitial aluminum for instance between the inter-metallics in the filter cake, when large quantities of inclusions are present, which results in a severe economic loss. Large quantities of inclusions are for instance present when the inclusions are formed on purpose, for instance when inter-metallics are formed in hypereutectic aluminum melts.